In a conventional rotary fuel pump, the angular position of a cam ring is adjusted by means of a servo advance arrangement. The advance arrangement includes an advance piston which is slidable within a bore and which cooperates, in use, with a cam arrangement of the fuel pump to adjust the timing of fuel delivery by the pump. A servo piston is slidable within a further bore provided in the advance piston and a light load sensing piston member is moveable relative to the advance piston against the action of a light load control spring. A servo control spring is engaged between the light load piston member and the servo piston and a control valve is operable to control the application of fuel to the light load piston member to adjust timing under light load conditions. Depending upon the engine load, the pressure of fuel acting on the load sensing piston varies, and the position of the load sensing piston changes. The movement of the load sensing piston results in movement of the servo piston which, in turn, causes movement of an advance piston. The movement of the advance piston causes movement of the cam ring, thereby adjusting the timing of fuel delivery by the pump.
The provision of a light load advance arrangement permits the timing of fuel delivery by the pump to be varied when the engine operates under a light load. The servo piston and the light load piston are arranged to define a light load control chamber for fuel, within which the servo control spring is arranged, a force due to fuel pressure within the light load control chamber acting on the light load piston member, in combination with the light load control spring, to determine the relative axial positions of the light load piston member and the advance piston.
The control valve is arranged to control the pressure of fuel within the light load control chamber by regulating the flow of fuel between the light load control chamber and a low pressure drain. The light load control valve arrangement typically includes a metering valve member which is angularly movable within a bore, the metering valve member being provided with a control edge which cooperates with a port provided in the bore so as to control the rate of flow of fuel out of the control chamber. The pressure of fuel within the control chamber determines the position of the light load piston member and this determines the maximum permitted level of advance. The position of the light load piston member also determines the relationship between engine speed and the rate of adjustment of timing of fuel delivery by the pump.
A problem can arise in fuel pumps of the aforementioned type in that the light load advance arrangement can cause the pressure of fuel delivered by the pump (referred to as ‘transfer pressure’) to be reduced as the engine load increases. It is desirable, however, to maintain a substantially constant transfer pressure as this improves the speed advance characteristic of the pump.
Another problem associated with the pump of the aforementioned type is that manufacturing variations in the control edge of the metering valve member forming part of the light load control valve arrangement can give rise to undesirable variations in the advance.
It is also known to provide the fuel pump with a cold advance arrangement to permit adjustment of fuel delivery timing depending on engine temperature. The pump includes a temperature control valve arranged to control the application of fuel to the servo or light load piston member depending on the temperature of the engine, thereby permitting adjustment of timing of fuel delivery to compensate for cold conditions.
Such arrangements do, however, suffer from the disadvantage that the cold advance arrangement can become unstable when speed advance is introduced.
It is an object of the present invention to remove or alleviate at least one of the aforementioned problems.